Precision machine vision inspection systems (or “vision systems” for short) can be utilized to obtain precise dimensional measurements of inspected objects and to inspect various other object characteristics. Such systems may include a computer, a camera and optical system, and a precision stage that is movable in multiple directions to allow workpiece inspection. One exemplary prior art system, that can be characterized as a general-purpose “off-line” precision vision system, is the commercially available QUICK VISION® series of PC-based vision systems and QVPAK® software available from Mitutoyo America Corporation (MAC), located in Aurora, Ill. The features and operation of the QUICK VISION® series of vision systems and the QVPAK® software are generally described, for example, in the QVPAK 3D CNC Vision Measuring Machine User's Guide, published January 2003, and the QVPAK 3D CNC Vision Measuring Machine Operation Guide, published September 1996, each of which is hereby incorporated by reference in their entirety. This type of system is able to use a microscope-type optical system and move the stage so as to provide inspection images of either small or relatively large workpieces at various magnifications.
Accuracies in the micron or sub-micron range are often desired in such systems. This is particularly challenging with regard to Z-height measurements. Z-height measurements (along the optical axis of the camera system) are generally derived from a “best focus” position, such as that determined by an autofocus tool. Determining a best focus position is a relatively complex process that generally depends on combining and/or comparing information derived from multiple images. Thus, the level of precision and reliability achieved for Z-height measurements is often less than that achieved for the X and Y measurement axes, where measurements are typically based on feature relationships within a single image. Recently, known techniques generally referred to as “structured illumination microscopy” (SIM) methods are being incorporated in microscopic measurement and inspection systems, in order to increase their measurement resolution and/or accuracy beyond the optical limits normally associated with simple imaging (e.g., to the micron and submicron level.)
Briefly, many SIM methods include projecting a pattern of light stripes onto a workpiece in a first image, and then shifting that pattern on the workpiece transversely to the stripes in a second image, and so on for a third image, or more. The resulting images may be analyzed according to known methods to improve the surface measurement resolution, as described in greater detail below. Such techniques may enhance X, Y, and/or Z measurements. However, the systems and methods used in known structured illumination pattern (SIP) generating subsystems (e.g., for forming and shifting the patterns) have so far limited the economy, versatility, and/or resolution and accuracy improvements of practical SIM systems in undesirable ways. In some methods of analysis, it is desirable for the stripes to exhibit a sinusoidal intensity profile across the stripes. In some systems, the SIM methods utilize sinusoidal patterns created by a digital mirror device (DMD) spatial light modulator (SLM) positioned in the projection path, and the optics project sinusoidal fringes onto a workpiece that is being measured. One advantage of such controllable digital SLM systems is that the size of the projected fringes can be adapted to be nearly optimum for any desired resolution, and/or imaging optics, and/or field of view. However, one disadvantage when the sinusoidal fringes are created with a digital SLM is that some of the higher harmonics of the fundamental sinusoidal frequency may be created and transferred by the optics through to the final light stripe images. The resulting height maps (HM) may contain effects from these higher harmonic artifacts, including Z-height errors which appear as approximately periodic “ripples”, which interfere with the accuracy of the measurements produced by the system. Thus, an improved method for economically utilizing SIM techniques based on controllable digital SLM's, while reducing the production of error artifacts (e.g., Z-height “ripples”) would be desirable.